1. Field of the Invention
The present invention relates to a high-strength steel plate resistant to strength reduction when processed by a stress relief annealing process (hereinafter, referred to as “SR process”) and resistant to cracking when processed by a welding process.
2. Description of the Related Art
Makers of large steel pressure vessels (tanks) are promoting on-site assembly of overseas tanks for cost reduction in recent years. It has been usual to complete a tank by carry out processes including a cutting process for cutting out steel workpieces, a shaping process for bending the steel workpieces, an assembling process for assembling the steel workpieces by welding, a SR process (local heat treatment) for processing some of the steel workpieces, and a final assembling process at the maker's plant and to transport the completed tank to an installation site.
There is a trend, in view of improving efficiency, toward building a tank by carrying out processes for cutting out workpieces, bending the workpieces to produce component members in the maker's plant, transporting the component members, building a tank on site by assembling the component members by welding and processing the entire tank by an on-site SR process.
As the method of building a tank thus changes, time for which the SR process is continued and the number of cycles of the SR process need to be increased from the view point of on-site welding techniques and safety. A fact that the component members of a tank are subjected to a SR process for a time between about 20, and about 30, hr in total needs to be taken into consideration in designing materials.
It is known that carbide grains contained in a steel agglomerate in large carbide grains remarkably reducing the strength of the steel when the steel is subjected to a SR process for such a long time. It has been a usual practice to suppress strength reduction due to long SR process and to prevent the coarsening of cementite grains by adding Cr to steels.
However, addition of Cr to a steel in a high Cr content deteriorates the weldability of the steel and often causes weld cracks to form. Under such circumstances, it has been desired to develop a high-strength steel plate, as a useful material for forming tanks, capable of minimizing strength reduction to the least possible extent and of ensuring satisfactory weldability even when the high-strength steel plate is subjected to along SR process.
Usually, Cr—Mo steel plates are used as steel plates capable of minimizing strength reduction due to processing by a SR process to the least possible extent. Such a Cr—Mo steel plate contains Cr in a high Cr content to suppress strength reduction due to a SR process and contains Mo to improve high-temperature strength.
A technique proposed in, for example, JP-A S57-116756 provides a tough and hard steel for pressure vessels basically containing 0.26, to 0.75% Cr and 0.45, to 0.60% Mo. This technique adds Cr to the steel to suppress the coarsening of carbide grains due to a SR process and to suppress strength reduction due to a SR process, the idea of which is the same as the foregoing basic idea. However, the weldability of this tough and hard steel is unsatisfactory because the tough and hard steel has a high Cr content.
A technique proposed in JP-A S57-120652, provides a high-strength steel for pressure vessels basically containing 0.10, to 1.00% Cr and 0.45, to 0.60% Mo. This technique intends to suppress the coarsening of Fe3C grains into large M23C6, grains due to processing by a long SR process by adding Cr. However, only high-strength steels having a Cr content of 0.29% or above are disclosed in JP-A S57-120652, and hence it is expected those high-strength steels are unsatisfactory in weldability.